A known method of forming a passivation film on a stainless steel surface is the wet process which forms the passivation film by immersing an electrolytically polished stainless steel into a solution of chemicals such as nitric acid at about 60.degree. C. The passivation film thus formed, however, contains iron oxide, though it gives a chromium-rich composition. In addition, the passivation film shows non-satisfactory corrosion resistance because of its thin layer, 20 .ANG. (angstroms) or less, and because of the presence of many pin holes.
Furthermore, that type of passivation film emits a considerable amount of moisture from its surface. The curve with the symbol of a closed circle in FIG. 9 shows the data of emission of moisture at room temperature from the surface of an oxide passivation film prepared by the wet process, determined by APIMS analysis. As seen in the figure, the oxide passivation film formed by the wet process can not fully release its moisture even after 100 minutes (min.) of emission. Since the oxide passivation film prepared by the wet process contains a considerable amount of moisture, it is inapplicable to semiconductor production equipment which requests emission-free condition. Therefore, that type of oxide passivation film further need to be subjected to heat treatment such as baking, which requires extra processing time.
As an alternative method of forming a passivation film emitting an extremely small amount of moisture, the present inventor proposed a dry process. The dry process is performed by directly reacting an electrolytically polished stainless steel with oxygen gas, then reducing the obtained iron oxide with hydrogen gas, followed by thermally treating the reduced material in an atmosphere of inert gas such as argon gas to form a passivation film consisting mainly of chromium oxide. FIG. 10 shows the block flow diagram of the dry process.
In FIG. 10, the number (1) denotes the baking step to remove surface moisture from the stainless steel, and (2) denotes the oxidation step which is conducted in an oxygen atmosphere. The film obtained in step (2) is an oxide passivation film consisting mainly of iron oxide. The number (3) denotes the reducing step to reduce the iron oxide and to obtain chromium oxide. The number (4) denotes the annealing step which is conducted in an inert gas atmosphere to convert the product into a passivation film consisting mainly of chromium oxide.
The curve with the symbol of a closed triangle in FIG. 9 shows the data of moisture emission from the surface of the passivation film prepared by the dry process. As seen in the figure, the moisture emission significantly decreased in the case of the dry process compared with the case of the wet process.
However, the above-described dry process to form a passivation film takes time because it conducts oxidation and reduction separately.
In response to this disadvantage, to solve the problems the present inventor provided another alternative method of forming an oxide passivation film consisting mainly of chromium oxide. According to the another alternative method, a stainless steel is subjected to electrolytic polishing or electrochemical buffing in a separate stage, and the pre-treated stainless steel is baked in an inert gas atmosphere to remove moisture from the surface thereof, then it is thermally treated at a temperature range of from 300.degree. C. to 600.degree. C. in an atmosphere of hydrogen gas or a mixture of hydrogen and an inert gas, which atmosphere further contains about 100 parts per billion (ppb) of oxygen or H.sub.2 O. (Japanese Patent Laid-Open No. 164377/1992; applied for by Tadahiro Ohmi.)
The other alternative technology forms an oxide passivation film which has a Cr/Fe ratio (atom ratio: same as in hereinafter) of one or more at the surface thereof, and also enables to form a passivation film having a layered structure consisting only of chromium oxide at the surface thereof. That type of passivation film gives less gas emission and has excellent corrosion resistence.
However, the technology has a limitation in terms of the thickness of the formed film, which is approximately 20 .ANG. at the maximum. Recent demand to increase the corrosion resistance requests the formation of a passivation film that provides a thicker layer consisting only of chromium oxide and has improved corrosion resistance.
In this respect, an object of the present invention is to provide a stainless steel having a corrosion resistance which is superior to the one prepared through prior art.
Another object of the present invention is to provide a method of forming an oxide passivation film having a layer consisting of a chromium oxide, which layer enables the formation of a passivation film consisting only of chromium oxide and having a thickness of 20 .ANG. or more at the surface thereof.